1. Field of the Invention
The present invention relates to the fabrication of GaAs monolithic microwave integrated circuits (MMICs), and, more particularly, to a method of improving adhesion of a dielectric layer on electrolytic gold plated electrodes.
2. Description of Related Art
Any exposed metal on the surface of a conventional MMIC is of serious reliability concern. The unprotected circuit elements risk being damaged during subsequent processing operations, including backside processing, on-wafer testing, wafer dicing, die pick-and-place, die attachment, off-chip electrical interconnect, and final cleaning and packaging. Accordingly, a dielectric coating, such as silicon dioxide, silicon nitride, silicon oxynitride, silicon carbide, or diamond, is often used to provide added protection against environmental attacks such as by chemicals, dirt, moisture, etc., and mechanical scratches, such as tweezer marks, handling tool marks, etc., making the MMIC chips more robust and more "silicon-like". A description of the formation and passivation of MMICs is given in U.S. Pat. No. 5,406,122, entitled "Microelectronic Circuit Structure Including Conductor Bridges Encapsulated in Inorganic Dielectric Passivation Layer", issued Apr. 11, 1995.
Gold-based metallization is widely used on MMIC chips. However, dielectric passivation coatings deposited directly on the gold surface often crack or peel when the part is subjected to thermal cycling. A thin (200 to 500 .ANG.) layer of titanium or chromium is commonly inserted between the gold-based electrodes and the dielectric passivation coatings of MMICs to promote dielectric-metal adhesion, which is critical to the reliability of the devices.
Titanium or chromium deposition is compatible with MMIC fabrication processes when evaporated or sputtered gold is used for electrode formation. However, thick (several micrometers) electrolytically-plated gold is required to form electrical crossovers to boost the power handling capability of electrical interconnects and to increase the Q-factor of the RF circuit elements. Titanium or chromium deposition is not compatible with the electrolytic gold plating process.
Accordingly, a need exists for preventing cracking or peeling of dielectric layers of MMIC devices formed on electrolytically-plated gold.